1. Field of the Invention
The invention relates to a process for determining the position of an influencing element with an inductive position sensor, with several coils which are arranged linearly or circularly in succession, with at least one capacitor, with an amplifier element, with at least one changeover switch and with an evaluation unit, one coil and capacitor at a time forming a tuned circuit and the tuned circuit and the amplifier element forming an oscillator as disclosed in published German Patent Application DE 101 30 572 A1 and corresponding U.S. Patent Application Publication 20030034785 A1.
2. Description of Related Art
Position sensors for determining the position of an influencing element are known in a host of embodiments and for a host of applications. These position sensors can, on the one hand, be classified according to whether the motion of the influencing element to be monitored is, first of all, linear motion so that a distance is to be detected by the position sensors, or whether the motion of the influencing element to be monitored is circular motion so that the position sensor monitors or ascertains the angle of rotation of the influencing element. Position sensors which detect a distance are often called distance sensors, while position sensors which detect an angle of rotation are often called angular resolvers.
In addition, position sensors can be classified according to their physical operating principle. For example, inductive, capacitive or optoelectronic position sensors are known.
The subject matter of this invention is a process for determining the position of the influencing element with an inductive position sensor, especially with an inductive displacement sensor with which a linear motion of the influencing element, i.e., a distance, can be measured. These known inductive displacement sensors have several coils, of which at least one coil is made as the primary coil and at least another coil is made as the secondary coil. The coils are generally built according to the transformer principle, so that each secondary coil is located laterally adjacent to the primary coil. The inductive coupling between the center primary coil and the two laterally arranged secondary coils is changed by the position of an influencing element which is made, for example, as a magnetically conductive rod and which is located in the area of the cylinder axis of the cylindrical coil system. These inductive displacement sensors are known from published German Patent Applications DE 43 37 208 A1 and DE 196 32 211 A1.
Published German Patent Application DE 31 02 439 A1 discloses an inductive displacement sensor with two magnetic circuits which are largely decoupled from one another, with two air-core coils, one core being able to dip into the first air-core coil, the instantaneous immersion depth of the core being inductively scanned, and a second core being located stationary in the second air-core coil. The position of the movable core can be determined by measuring the inductance ratio of the first air-core coil to the second coil.
Published German Patent Application DE 42 13 866 A1 discloses an inductive rotary sensor in which several coils are located next to one another on a base plate such that the coils can acquire several measurement values at the same time, by which relatively exact extrapolation of the rotor position is possible. The individual coils are permanently connected to an oscillator stage, the outputs of the oscillator stages being supplied in parallel to the evaluation unit. The position of the evaluation unit is detected by means of pattern analysis of several frequency values which are measured at the same time. However, this type of evaluation can only be conditionally used for high-speed applications as a result of the large amount of information.
The disadvantage of the known inductive displacement sensors is that, on the one hand, the structural length of the displacement sensor is clearly longer than the maximum monitored path of the influencing element so that, at a given path length to be monitored, a displacement sensor up to 100% longer is necessary. This is especially undesirable wherever a limited installation space is available. On the other hand, in the known inductive displacement sensors, the attainable measurement accuracy is often not sufficient or it can only be improved by increased circuit complexity.
This problem is solved in the inductive displacement sensor of published German Patent Application DE 101 30 572 A1, and corresponding U.S. Patent Application Publication 20030034785 A1, in that the individual coils or the individual oscillators are selected in succession by a changeover switch, thereby connecting the individual coils in succession to the capacitor, and in that the evaluation unit measures the change of the impedance of the coil selected by the changeover switch or of the tuned circuit selected by the changeover switch as a function of the position of the influencing element relative to the respective coil.
It is also possible to connect the individual coils in succession not only to a capacitor, but to a fixed, defined tuned circuit. If this fixed tuned circuit is connected to the amplifier element, the circuit has a continuously oscillating oscillator to which only one other (measurement) coil at a time is connected. This has the advantage that oscillation build-up of the tuned circuit or of the oscillator is not necessary.
Although it has been stated above that the individual coils are connected in succession to the capacitor, or to the fixed, defined tuned circuit, it is not meant that the individual coils must be selected in succession by the changeover switch according to their three-dimensional arrangement. Basically, it is also possible to select any coils in time succession by the changeover switch.
By using several coils in succession, the coils being arranged in succession in the direction of the position of the influencing element which is to be ascertained, and the evaluation unit measuring in succession the change of the impedance of each coil or each tuned circuit as a function of the position of the influencing element by the changeover switch, an inductive displacement sensor can be implemented with an overall length which is slightly greater than the total length of the distance which is to be monitored.
In the process of published German Patent Application DE 101 30 572 A1 and corresponding U.S. Patent Application Publication 20030034785 A1, the position of the influencing element can be very precisely and reliably detected by means of an inductive position sensor by the process having the following steps:
selecting one coil or oscillator at a time by the changeover switch by connecting the individual coils in succession to the capacitor, and
measuring the impedance of the coil selected by the changeover switch and of the tuned circuit selected by the changeover switch by the evaluation unit as a function of the position of the influencing element relative to the coil,
the aforementioned steps being repeated until all coils have been selected in succession by the changeover switch, i.e., have been connected in succession to the capacitor and the impedance of all coils has been measured by the evaluation unit.
However, the known process has the disadvantage that, under certain circumstances, it does not have sufficient measurement speed or reaction time at the desired high accuracy.